Sync eng/common directory with azure-sdk-tools for PR 12758 (#43911)

* add reference doc for adding client customizations in TypeSpec

* Update eng/common/knowledge/customizing-client-tsp.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Mariana Rios Flores <mariari@microsoft.com>

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Co-authored-by: Christopher Radek <Christopher.Radek@microsoft.com>
Co-authored-by: Christopher Radek <14189820+chrisradek@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Mariana Rios Flores <mariari@microsoft.com>

Author: 5 people